1. Field of the Invention
The present invention relates to an ion source for generating ions by exciting a gas using an electron beam and, more particularly, to an improvement in the electrode of an ion source.
2. Description of the Related Art
An ion implantation system is widely used to dope impurity ions into a semiconductor wafer in the manufacturing process of a VLSI. An ion implantation system is required to control a desired ion implantation amount and depth with high precision. Various types of ion sources are available for an ion implantation system so that ions having various energy levels and current densities can be produced in accordance with the purpose of a process.
For example, an electron beam excited ion source includes a first chamber for generating a first plasma (argon plasma), and a second chamber for generating a second plasma (BF.sub.3 plasma). The first chamber is constituted by a main chamber for generating thermoelectrons, and a sub-chamber in which a discharge gas (Ar gas or the like) is injected together with the thermoelectrons through a nozzle upon starting up. The second chamber is partitioned from the first chamber by an electrode in terms of energy potential and serves to ionize a source gas (BF.sub.3 gas or the like) by electron discharge/excitation.
In the electron beam excited ion source, thermoelectrons are generated from a filament, and an Ar gas is introduced into the first chamber while a voltage is applied between the filament and the electrode. When the thermoelectrons are caused to pass through the nozzle together with the Ar gas, gas molecules are dissociated from each other by discharge, and an argon plasma is produced.
A through hole (electron beam passing hole) is formed in the electrode. When a potential is applied between the electrode and a chamber side wall, only electrons are extracted from the first plasma into the second chamber through the through hole.
The electrons are then vertically guided in the second chamber by a magnetic field. The source gas (BF.sub.3 gas or the like) is introduced into the second chamber in a direction perpendicular to the propagation direction of the guided electron beams, thus exciting the source gas by PIG discharge and generating a BF.sub.3 plasma.
Desired ions are extracted from the second plasma and are guided to a target (semiconductor wafer) through a guide tube so as to cause the ions to collide with the target. According to such an electron beam excited ion source, high-current-density ions can be obtained.
With a recent increase in packing density of a semiconductor device, a demand has arisen for an increase in ion production efficiency in an ion source. If the ion production efficiency is increased, a large amount of ions can be generated at low cost. This increases the throughput and decreases the running cost. In order to increase the ion production efficiency, the number of passing electrons may be increased by increasing the diameter of the electron beam passing hole of the electrode.
In the above-mentioned electron beam excited ion source, however, if the diameter of the electron beam passing hole of the electrode is increased, the first and second plasmas tend to communicate with each other through this hole. This makes the second plasma unstable. As a result, the ion production efficiency is decreased.
If the diameter of the electron beam passing hole of the electrode is reduced, the density of gas molecules passing through the hole is increased, and gas molecules collide with electrons in the hole, thus causing local discharge and generating a plasma. Owing to this new plasma, the first and second plasmas tend to communicate with each other. For this reason, a desired potential cannot be applied to an electron beam.
Each of the first and second chambers is constituted by combination of conductive and insulating members excellent in durability. However, since a plasma is produced in each chamber, the conductive member of each chamber is damaged due to the effect of the plasma such as etching and sputtering, and abraded fine particles of the conductive member are attached to the insulating member, thus causing an insulation fault.